This invention relates to dry etchers for etching layers on a semiconductor wafer. Dry etching offers the important manufacturing advantage of eliminating the handling, consumption, and disposal of the relatively large quantities of dangerous acids and solvents used in wet etching processes.
Popular dry etch techniques include plasma etching and Reactive ion etching (RIE). These techniques are standard etching techniques used in the industry for anisotropic etching. In the RIE mode, the wafers are placed directly upon an electrode connected to a radio frequency (RF) power generator. Neutral chemical reactive species and ions are formed in a plasma discharge between the electrodes. These ions energetically bombard the wafer to help cause a directional etch.
In plasma etching, the wafer is placed on an electrode which is connected to the ground. The radio frequency power source produces chemically reactive species and ions. Since an electrical potential difference exists between the plasma and the grounded electrode on which the wafers are placed, the ions are still attracted to the wafers. Thus, even in plasma etch mode, wafers are subject to energetic ionic bombardment although usually to a much lesser degree than in the RIE mode.
In both RIE and in plasma etch modes the bombarding ions and the chemically reactive species can form an anisotropic or directional etch onto the wafers. In anisotropic etching, ions bombard the layer to be etched and the chemically reactive species aid in the etching.
In reactive ion etching, various forms of radiation damage can be formed on the materials being etched because of the energetic bombardment of ions. This radiation damage can include electron traps in the gate oxides, which if not annealed out, can cause shifts in the threshold voltages of metal-oxide-silicon (MOS) devices; displaced atoms and implanted atoms in the surface due to ion bombardment; and under some etching conditions, the destruction of the gate oxide.
An etching technique typically used for isotropic etching is downstream etching. Isotropic etches etch the wafer in all directions at substantially the same rate and therefore isotropic etches are not directional.
Downstream etching is discussed in "A New Chemical Dry Etching" Y. Horiike and M. Shibagaki, Proceedings of the 7th Conference on Solid State Devices, Tokyo, 1975. FIG. 1 is a schematic diagram of a prior art radio frequency (RF) downstream etcher. A gas supply 2 provides gases to a container 4 defining a chamber 6. A radio frequency generator 8 is connected to electrodes 10 and 12. The electric signal between the electrodes 10 and 12 through the gases in the chamber 6 produces a plasma discharge that forms chemically reactive species and ions. Since the wafer 14 is some distance from the electrodes 10 and 12, the wafer will not suffer radiation damage. The chemically reactive species and ions created in the plasma discharge between electrodes 10 and 12 flow towards the wafer 14. Due to the distance of the wafer from the electrodes, the ions will recombine before reaching the wafer. Since the chemically reactive species are longer lasting, the etching at the wafer 14 is caused by the chemically reactive species. The gases in the chamber 6 are removed by ports 16 and 18 connected to a pump 20.
Downstream etching avoids the radiation damage to the wafer which is often found in reactive ion etching. The downstream etcher, however, cannot produce a truly anisotropic profile due to the lack of directional ion bombardment.